Ask the Expert: September 2015

It is estimated that a significant proportion of individuals infected with HIV have yet to be diagnosed and therefore serve as a reservoir for continuous infection. Public health efforts to diagnose these unknown cases are critical because we now have effective treatment protocols that prevent the development of adverse pathologic complications and enable patients to have an almost normal life span. Early diagnosis also curtails the spread of the disease in the general population.

What are the different types of HIV?

Understanding the basic biology of the virus and the heterogeneity in its grouping is critical to the developing of HIV diagnostic assays and appropriate interpretation of results. The virus is known to have two phylogenetic types, HIV-1 and HIV-2, with additional groups within each type. HIV-1 has four groups: M, O, N, and P. Group M, the predominant group in the U.S., is further divided into about 11 subtypes (A, B, C, D, F, G, H, J, K, CRF1, and URF2). The CRF1 subtype also has five sub-subtypes (A1, A2, A3, F1, and F2). HIV-2, which is predominantly seen in Africa, has eight well documented groups (A–H), but only groups A and B are epidemic. These diverse HIV strains have genotypic and phenotypic characteristics that produce specific host immunologic responses and antibodies. These antibodies, in turn, serve as the markers that HIV assays target.

When do different serum markers for HIV infection appear and how does this time sequence affect test results?

It takes 1–9 months for antibodies to the viral particles to appear in serum at measurable quantities (with the average time being 2–3 months), while viral nucleic acid and protein increase to quantifiable levels within 1–2 weeks post-infection. Therefore, the first diagnostically useful markers in human serum that should be leveraged for diagnosis are the viral nucleic acid (RNA) and proteins (i.e., p24) that gradually increase in quantity as the virus replicates. These viral particles reach their peak levels by day 28 after infection. Typically, the p24 viral particle then returns to undetectable levels by day 60 while HIV RNA decreases slightly but remains at detectable levels throughout the course of infection. The first detectable levels of antibodies against the viral particles are noted beginning at day 21 and peak at about 45 days post-infection. After this, the antibodies also remain elevated indefinitely. Due to the varying timelines and windows of detection for these markers, which are used for different HIV assays, it is not atypical to get false-negative HIV test results if the assay is not appropriately interpreted relative to the time sequence of the marker employed in the assay. For example, an immunoassay for HIV IgM and/or IgG will yield a false negative during the window period when no antibodies can be detected even when HIV is present.

How have immunoassays for HIV been optimized to improve the time period when they can detect the virus?

First-generation HIV screening immunoassays were capable of detecting infection at approximately day 58 after exposure to the virus. Subsequent refinement of these immunoassays led to second-generationtests that could detect infection at about day 42 post-infection, and with the introduction of third-generation immunoassays, the post-infection detection time decreased to days 21–25. Most recently, fourth- and fifth-generation HIV screening tests that target both HIV proteins (p24) and human antibodies (IgG and IgM) to the HIV proteins have decreased the postinfection detection time to 10–14 days. The continuous optimization of HIV assays has significant public health implications and could play a key role in limiting the spread of the virus.

Anthony O. Okorodudu, PhD, MBA, DABCC, FACB, is a professor of pathology and director of the Clinical Chemistry Division and UTMB/ Correctional Managed Care Laboratory at University of Texas Medical Branch in Galveston. +EMAIL: aookorod@utmb.edu